This invention relates to a means for controlling the generation of a plurality of electrical generating units such as those under regulation from a central power station. More particularly, this invention relates to a system for controlling units which have an inherently slow response when they are to be controlled with fast responding units. The slow units are, for example, the coal fired units which require pulverization of the coal before it is fed into the boiler. This type of unit is inherently slow in responding to control requiring a change in firing rate because of the slow rate of change at which the pulverizer will operate.
When units having a slow response are controlled by a control system which also controls fast responding units, it has been found that if the controller is tuned to have the proper characteristics for controlling the fast units, the slow units are improperly controlled. The improper control occurs due to the fact that the generating unit will respond quickly to the control signals related to the controlled variable (area control error) changing the manipulated variable (steam flow), but the change in firing rate which is necessary to support that change in flow at the proper steam pressure will not occur as quickly. Thus, there is a rapid increase in steam flow without a comparable change in fuel firing rate due to the slowness of the pulverizer. This causes the steam pressure of the unit to fall and the pressure control for the boiler to call for an increase in firing rate. The result is that the two controls (pressure and load) call for an increase in firing rate in such a way that the pressure control is likely to cycle as it fights the master controller which is controlling load in response to area control error (ACE). This is demonstrated by FIG. 1 in which a ramp change in megawatts output from a coal fired unit is shown to follow closely the response of the governor. As a result of the load change, the throttle pressure drops and as it recovers, in response to the pressure control increasing the fuel flow, the governor control and the pressure control start to fight each other and the result is a steady generator output with a cycling of the governor and pressure control.
In the past, control problems of this type have been solved by using process models to determine the expected generation change. That approach has only had limited success because of the great difficulty involved in providing an adequate model.
It is an object of this invention to provide a control system for load distribution control which will prevent such interaction.